Measurement of beam position monitor using HOM couplers of superconducting cavities

The offset beam from the axis induces the HOMs in the cavities. These HOMs in superconducting cavities are usually damped by HOM couplers to suppress the beam instability. The induced HOM field of dipole mode is proportional to the beam offset and can be used to measure the beam position inside the cavity. Measuring the HOM power by scanning the beam permits to estimate the beam position using a known function of the HOM power. The steering magnet was installed to the JAEA superconducting ERL-FEL to vary the beam position. The beam position in the cavity was estimated with the measured HOM power from the HOM coupler.